Strontium desorption from bacteriogenic iron oxides (BIOS) subjected to microbial Fe(III) reduction

The effect of microbial reduction of bacteriogenic iron oxides (BIOS) on the release of sorbed Sr was examined using a naturally low-Sr BIOS (n-BIOS) collected from Chalk River, Canada, and a naturally high-Sr BIOS (Äspö-BIOS) collected from Äspö, Sweden. EXAFS analysis suggested that Sr was bound to the Äspö-BIOS as an outer-sphere complex of hydrated Sr2+. During reduction, Sr desorption from n-BIOS was concomitant with the production of Fe(II), resulting in solubilization of 80% of the total Sr. In contrast, nearly 100% of the Sr was solubilized from Äspö-BIOS, although Sr desorption was delayed, relative to Fe(II) production. When n-BIOS were artificially saturated with Sr, there was a significant decrease in the rate and extent of Fe(III) reduction and an induced delay in Sr desorption (similar to that observed for Äspö-BIOS). Saturation of synthetic hydrous ferric oxide with Sr had no such effect on Fe(III) reduction rate, although a delay in Sr desorption was again observed. In all instances, the levels of desorbed aqueous Sr remained constant, suggesting that re-sorption or co-precipitation of Sr with secondary mineral precipitates (vivianite) did not occur under the experimental conditions. In situ porewater analysis of the Chalk River BIOS indicated positive correlations between soluble-phase concentrations of Fe(II) and Sr at depth, suggesting that anaerobic reduction of Fe(III) releases Sr back into the groundwater flow at the site. Further, in areas where BIOS deposition is occurring within the groundwater discharge zone, levels of soluble Sr are 2 to 3 times lower than in areas where BIOS is absent. The data indicate that phase partitioning of Sr within BIOS is governed largely by Fe oxidation state, and that BIOS will act as effective sorbents of Sr as long as oxidizing conditions are maintained. However reduction of the BIOS will likely result in significant desorption and remobilization of the bound Sr.